Under normal circumstances the human body is exceptionally efficient at regulating a constant internal temperature. However, increased workload compounded by environmental factors such as air temperature, radiant heat sources, and humidity may stress the body's ability to safely regulate its internal temperature. Heat stress is a potentially dangerous build up of heat within a body and is a hazard faced by many workers and athletes. The use of necessary protective apparel when working in hot environments puts such workers at an increased risk of heat stress. On the other end of the temperature spectrum, workers in cold environments are at risk of an unsafe decrease in the body core temperature known as hypothermia.
The current industry practice to limit the potential hazard of thermal stress includes controlling work/rest cycles based on environmental conditions. Such guidelines are conservative estimates based on average workers and vary based on a person's age, weight, physical fitness, degree of acclimation, use of alcohol or drugs, various medical conditions, clothing being worn, and other individual-specific factors. Thermal stress, including both heat stress and hypothermia, is indicated by several physiological changes and has been studied extensively in the past. Many thermal stress indicators involve consideration of the environmental factors and individual-specific factors as discussed above and comparing them with known tabulated data. However, such measurements and use of reference materials is not necessarily convenient or practical in the average dynamic work environment.
One key indicator for determining the onset of thermal stress (either heat stress or hypothermia) is the true core body temperature. Multiple safety standards agree that the body core temperature should not be allowed to exceed 38° C. for extended periods of time, nor should the core temperature be allowed to increase at a rate of much greater than 1° C. per hour. Similarly, the onset of hypothermia occurs when the body core temperature drops below 35° C. Having a core body temperature outside the range of about 35° C. to about 38° C. can result in the failure of various systems of the body and may ultimately result in death.
There are several known methods to measure/estimate core temperature. Invasive techniques used include rectal probes, esophageal catheters or capsules that are swallowed. When patients are catheterized, blood temperature or urine temperature in the bladder may also be used as a good indicator of core body temperature. While such invasive measurement methods work well for patients in a controlled environment, such techniques are not feasible for use with workers in a comparatively uncontrolled working environment or for an athlete undertaking their particular activity. Such invasive methods are even less practical in situations where continuous monitoring the core temperature of such a worker or athlete is desired.
Several minimally invasive methods of estimating the core body temperature from skin temperature measurements have been developed. Due to the differences between the skin temperature and the core body temperature, such methods have to modify the measured skin temperature to estimate the true body core temperature. Some estimates modify the measured skin temperature utilizing other environmental data such as ambient temperature and ambient humidity, either measured by the device or inputted by the user. Other estimates of core body temperature require input or acquisition of user-specific data. For example, a series of baseline measurements may be taken over a period of time to calibrate the skin temperature measurements for the particular user. Other estimates are left in insulated contact with the skin of the user until a presumed equilibrium of body core temperature and skin temperature is reached in the region of insulated contact. All of such estimates of body core temperature are often adequate for monitoring the temperature of a patient in the controlled care environment.
However, such estimates of core temperature are user-specific and are not practical for the working environs, and under the conditions, in which a worker or athlete may be at risk for thermal stress and would particularly benefit from such monitoring. For example, as a worker (or athlete) exerts himself or herself, their body temperature may rapidly increase. Their body will attempt to regulate the internal body temperature through various methods including increasing perspiration for the purposes of evaporative cooling. In such situations of rapid temperature change and cooling of the skin by perspiration, the assumptions underlying existing models of estimating core temperature from skin temperature are broken. Thus, estimates of core body temperature may become more inaccurate in situations of rapid temperature change and increased subject perspiration; the very situations in which such thermal stress monitoring is most needed.